Vehicle steering columns are assembled from a number of universal or Cardan joints. Typically, an intermediate axis of a steering column has a Cardan joint at each end: one Cardan joint is connected to the motion input of the steering gear case, and the other Cardan joint is connected to the upper column. In order for the steering mechanism to function properly, the length of the intermediate axis must be variable. The variability of the intermediate column is adapted to oscillations of the front of the vehicle. These oscillations are due to the profile and the surface condition of the road. A variable length of the intermediate axis is also required in order to facilitate the assembly of the rack pinion, and to absorb motions of a front impact.
There exist many sliding shaft coupling devices, which transfer torque between the shafts by using splines, which are provided on each of the two shafts with curved profiles. However, this type of device develops wear over the life of the vehicle. In order to delay the occurrence of this wear, many prior art sliding shaft coupling devices use very tight tolerances between the shafts during manufacture. The tight tolerancing requires a relatively large axial force for proper assembly, hence a longer assembly time and more labor required to assemble the coupling device.
There also exist coupling devices which use plastic injection on splined portions belonging to a male metal shaft and a female metal tube. This solution poses problems for absorbing axial movements when the torque on the shaft is high, as the sliding forces increase proportionally with the friction forces between both splined portions. Moreover, the wear of the plastic parts leaves play between the shafts, which causes deteriorated performance of the coupling device.
As a last solution, rolling elements and strain springs may be introduced between the shaft and the tube. This solution is satisfactory for the sliding, under a torque with a relatively low axial force, but it poses problems in terms of angular rigidity because the torsional stiffness is directly proportional to the stiffness of the pre-tensioning of the springs.
The axial variability of the column should be accurate so as to allow proper absorption of the axial movements. In current coupling devices, the axial force is a function of the torque to be transmitted. There is an increased sliding force with an increased torque, which is related to the friction coefficient. With a sudden axial release, jerks can occur. These jerks are detrimental to maintaining proper adjustment of the axial sliding while maintaining reduced rotational play. The jerks are further harmful to driving and handling performance.
There is a need for an improved coupling device with a variable length that overcomes the disadvantages of the prior art. Further, a function is required for transmitting the rotary movement between both shafts and for the torque necessary for maneuvering the vehicle.